The width of the decay \(X_{0}\rightarrow D^{-}K^{+}\)  is determined by the
strong coupling \(G\)  corresponding to the vertex \(X_{0}D^{-}K^{+}\) . We are
going to calculate \(G\)  using method of the QCD sum rule on the light-cone
[1]}, [2]} and methods of the soft-meson
approximation [3]}. To this end, we start from analysis of
the correlation function [2]}, [5]}
\(\Pi (p,q)=i\int d^{4}xe^{ipx}\langle K(q)|\mathcal {T}\lbrace J^{D}(x)J^{\dag }(0)\rbrace |0\rangle , \) 
